net: rename CNetAddr scopeId to m_scope_id, improve code doc

Summary:
This is a backport of core#19951 [2/2]
https://github.com/bitcoin/bitcoin/pull/19951/commits/f36887fa47b42af60f8a06a3995baca7c73ca310

Test Plan: ninja all check-all

Reviewers: #bitcoin_abc, majcosta

Reviewed By: #bitcoin_abc, majcosta

Differential Revision: https://reviews.bitcoinabc.org/D10426